This invention relates generally to semiconductor devices and more particularly to semiconductor devices adapted to operate with high levels of microwave power.
As is known in the art, it is frequently desirable to use microwave diodes in a variety of high power applications. When used in such applications the diode may be mounted to a pedestal shaped heat sink used to extract heat from the diode. Further, in the fabrication of individually mesa shaped diodes, prior to dicing the wafer into individual diodes a wafer used to form such diodes has disposed on the entire back surface thereof a thick plated heat sink. After separating the diodes this thick plated heat sink is mounted to the pedestal shaped heat sink. When the material of the pedestal heat sink has a higher thermal conductivity than the material of the thick plated heat sink forming part of the mesa shaped diode, it is desirable to minimize the thickness of the thick plated heat sink in order to minimize the thermal resistance of the diode. However, a thick plated heat sink generally is required to provide structural integrity to the wafer after the diodes have been formed into mesas. This is because after mesa definition, the wafer of mesa shaped diodes is supported only by the gold plated heat sink structure and further photolithographic steps and processing steps are still generally required after the mesa definition. Therefore, if the heat sink is too thin the structure supporting the mesa shaped diodes may flex, bend, or crease making it difficult to handle during the additional photolithographic and processing steps, thus resulting in lower device yields.
As also known in the art, a critical step in the assembly of microwave power diodes is the step of mounting the diode in a microwave package and the subsequent interconnection of the diode to the package terminals. Generally, the interconnection is accomplished by soldering the plated heat sink into the package to form a first contact and using a wire preform attached to the top of the mesa shape diode as a second contact. The wire preform is typically ultrasonically bonded to the top of the mesa shaped diode. This packaging technique has several short comings, however, particularly when applied to millimeter wave diodes. The mesas shaped millimeter wave diodes are comparatively small and fragile compared to X-band diodes, for example. This imposes several contraints on the bonding operations and the preform lead wires: bonding forces and ultrasonic power must be kept to a minimum, often producing a bond of questionable strength, bonding tool diameters must be small in order to avoid excessive force on the mesas tops and damage by off center bonds, and lead preforms are difficult to precisely shape leading to unpredictable package parasitics, resulting in degraded device performance. Moreover, lead bonding is a costly and time consuming process requiring a high degree of bonding operator skill.
As is also known in the art, it is frequently desirable to use a plurality of individual mesa-shaped diodes with the mesa shaped diodes having a total area equal to an equivalent single mesa shaped diode. A plurality of individual diodes adapted to operate at X-band may be individually mounted in a package. However when the diodes are designed to operate at multimeter wavelengths, for example, their size is so small that mounting them individually to provide a plurality of diodes in such a package is difficult.